A team of scientists led by Arizona State University has discovered new fossils in Africa that suggest Australopithecus and the oldest specimens of Homo lived in the same place at the same time between 2.6 million and 2.8 million years ago. The researchers identified a new species of Australopithecus not previously found.
The Ledi-Geraru Research Project, headed by ASU scientists, has already produced significant finds, including the oldest known member of the genus Homo and some of the earliest Oldowan stone tools.
The team determined that recently uncovered Australopithecus teeth from Ledi-Geraru represent a new species rather than belonging to Australopithecus afarensis, famously known as “Lucy.” This supports previous findings that Lucy’s species does not appear in fossil records after 2.95 million years ago.
“This new research shows that the image many of us have in our minds of an ape to a Neanderthal to a modern human is not correct — evolution doesn’t work like that,” said Kaye Reed, ASU paleoecologist. “Here we have two hominin species that are together. And human evolution is not linear — it’s a bushy tree; there are life forms that go extinct.”
Reed is a research scientist at ASU’s Institute of Human Origins and President’s Professor Emeritus at the School of Human Evolution and Social Change. She has co-directed the Ledi-Geraru Research Project since 2002.
The discovery includes 13 fossil teeth from both Homo and what appears to be a new Australopithecus species. In 2013, Reed’s team also found a jawbone from the earliest known Homo specimen at this site.
“The new finds of Homo teeth from 2.6- to 2.8-million-year-old sediments — reported in this paper — confirms the antiquity of our lineage,” said Brian Villmoare, lead author and ASU alumnus.
“We know what the teeth and mandible of the earliest Homo look like, but that’s it. This emphasizes the critical importance of finding additional fossils to understand the differences between Australopithecus and Homo, and potentially how they were able to overlap in the fossil record at the same location.”
The researchers noted that more fossils will be needed before they can formally name this newly identified species.
To determine their age, geologists analyzed volcanic ash layers containing feldspar crystals above and below where fossils were found. Christopher Campisano, an ASU geologist involved with dating these deposits, explained: “We can date the eruptions that were happening on the landscape when they’re deposited… And we know that these fossils are interbed between those eruptions, so we can date units above and below the fossils. We are dating the volcanic ash of the eruptions that were happening while they were on the landscape.”
Understanding when these hominins lived helps reconstruct their environment millions of years ago—a time when rivers crossed vegetated plains into shallow lakes—contrasting with today’s arid badlands at Ledi-Geraru.
Ramon Arrowsmith, another ASU geologist who has worked with Ledi-Geraru since 2002, described its value for studying early human evolution: “It is a critical time period for human evolution, as this new paper shows,” he said. “The geology gives us the age and characteristics of the sedimentary deposits containing the fossils. It is essential for age control.”
The research team continues analyzing tooth enamel to learn about ancient diets and ecological interactions between early Homo and Australopithecus at Ledi-Geraru.
“Whenever you have an exciting discovery, if you’re a paleontologist, you always know that you need more information,” Reed said. “You need more fossils. That’s why it’s an important field to train people in and for people to go out and find their own sites and find places that we haven’t found fossils yet.
“More fossils will help us tell the story of what happened to our ancestors a long time ago — but because we’re the survivors, we know that it happened to us.”
The findings appear in Nature under “New discoveries of Australopithecus and Homo from Ledi-Geraru, Ethiopia.” Many contributors are current or former members of Arizona State University.
